Electronic Devices - Special Purpose Junction Diodes (Zener, LED, Photodiode)

A Zener Diode is a heavily doped $p-n$ junction diode designed to operate specifically in the reverse breakdown region. Because of heavy doping, the depletion layer is very thin ($\\approx 10^{-6} \text{ m}$), leading to a high electric field even at low voltages.

The Zener Breakdown occurs when the high electric field across the junction pulls electrons out of the covalent bonds, creating a large number of charge carriers. It is used for Voltage Regulation because the voltage across the diode remains constant ($V_Z$) even if the current through it changes significantly.

A Light Emitting Diode (LED) is a forward-biased $p-n$ junction that emits spontaneous radiation. When the diode is forward biased, electrons from the $n$-side and holes from the $p$-side move toward the junction and recombine. The energy released during recombination is emitted as photons if the energy gap $E_g$ is in the visible or infrared range.

For an LED, the wavelength of light emitted is given by $\lambda = \frac{hc}{E_g}$. The color of light depends on the semiconductor material used (e.g., $GaAsP$ for red/yellow light).

A Photodiode is a special purpose $p-n$ junction diode operated in reverse bias. It is used to detect optical signals. When light of frequency $\nu$ such that $h\nu > E_g$ falls on the junction, it creates electron-hole pairs near the depletion region, increasing the reverse saturation current.

In a photodiode, the reverse current is directly proportional to the intensity of incident light. It is operated in reverse bias because the fractional change in minority carrier current is much more easily measurable than the fractional change in majority carrier current.